AppInstaller Failed Installation

I've been using the AppInstaller app to 'side load' my LOB UWP app through an IIS web page. This has been working quite well until mid-July. However, now the app will no longer auto-update, and a fresh install shows a super informative error of:

'App installation failed with error message: Catastrophic failure (0x8000ffff)'

I've checked for clues in the Microsoft-Windows-AppXDeploymentServer/Operational event log, and there are several errors in the log, but they're equally helpful with text of:

'AppX Deployment operation failed for package ..... with error 0x80073CF9. The specific error text for this failure is: Unspecified error'

Using the Add-AppDevPackage.ps1 does install the application, but using this method does not auto-update, and visiting each device for each user is not desirable.

How can I debug installation problems for AppInstaller App?